1. Field of the Invention
The present invention relates to an improvement of a mobile body navigation apparatus having a capability for recording and displaying position data representing all or part of a route which has been traversed by a mobile body such as a motor vehicle, or for transmitting position data representing the traversed route and current position by radio in the event of an emergency.
2. Description of the Prior Art
Prior art types of navigation apparatus for mounting on a mobile body such as a motor vehicle are known, having a traversed route display function for displaying at least a part of the route which has been traversed by the vehicle. Successively calculated sets of position data for generating this traversed route display, i.e., data expressing successively attained positions of the vehicle, are recorded temporarily in a data storage device, and read out therefrom for use in generating the route display. The present invention relates in particular to a hybrid vehicle navigation system, which utilizes a combination of systems for deriving both absolute positions and relative (i.e., dead-reckoning) positions. With such a system, absolute position fixes are periodically obtained, based on information conveyed by radio signals received from a navigation signal source such as the GPS (Global Positioning Satellite) system, so long as satisfactory signal reception continues. In the intervals between obtaining these absolute position fixes, relative (dead-reckoning) positions of the vehicle are periodically calculated, based on information provided by relative position sensors of the vehicle such as a speed sensor for detecting amounts of distance traveled by the vehicle since the most recent absolute position fix, and a direction sensor such as a vibration gyroscope for detecting changes in the vehicle heading direction. Each position may be specified only as a lateral position (i.e., a pair of longitude and latitude values), or may also specify altitude.
The amount of data storage capacity required for the route display function of such an apparatus is determined by the maximum length of route which is to be displayed, that is, the number of vehicle positions that must be stored to represent that length of route. It would be desirable to enable that maximum length of displayed route to be increased without a corresponding increase in the data storage capacity requirements and without significant lowering of accuracy of the displayed route.
Furthermore, in recent years, types of emergency vehicle dispatch system have been developed whereby when an emergency condition such as a traffic accident occurs for a motor vehicle, the dispatch of emergency services vehicles can be facilitated by transmitting data expressing the approximate position of the accident site directly from that vehicle, for example to an emergency services center. This done by using position data derived by a vehicle-mounted navigation system and transmission of the data from the vehicle by radio communication via, for example, a portable telephone equipped for digital data transmission. The emergency services center can thereby rapidly dispatch the appropriate emergency service vehicle to the position of the accident.
If only data expressing a single position (i.e., the approximate final position of the vehicle) are transmitted to the emergency services center, it may not be possible for the position of the accident site to be specified with sufficient precision. For that reason, it is preferable to transmit data expressing the most recently traversed part of the route that has been taken by the vehicle which is at the accident site, i.e., a sequence of positions extending up to the most recent position that has been derived by the vehicle navigation system. In that way, there is a greater likelihood that the emergency services center will be able to accurately judge the accident position based on the received data. In particular, when the route lies within a town or city, it becomes possible for the emergency services center staff to perform map-matching to accurately determine the accident position, i.e., the shape and orientation of the received route can be compared with the arrangement of streets in a map, to thereby reliably ascertain that the requisite position (the point of termination of the vehicle route) is on a specific street.
However due to the large amount of data which are necessary to express such a traversed route, a substantial amount of time will be required to transmit the necessary data. Hence, the problem arises that the time required to notify the necessary information to an emergency vehicle dispatch center may be excessively long. Conversely, if it is attempted to overcome this problem by transmitting only a small amount of position data, then it becomes difficult for the emergency vehicle dispatch staff to accurately judge the actual position of the accident site, so that effective dispatch of assistance may not be possible.
It is a first objective of the present invention to overcome the above problems of the prior art, by providing a position data decimation method and position data decimation apparatus for use with a vehicle navigation apparatus having a traversed route display function, whereby the amount of data required to be recorded for use in displaying all or part of a traversed route can be substantially reduced.
It is a second objective of the invention to provide a position data decimation method and position data decimation apparatus for use with a vehicle navigation apparatus having a capability for radio transmission of position data expressing a traversed route, whereby the amount of data required to be transmitted for expressing all or part of a route traversed by the vehicle can be substantially reduced.
The invention provides two basic methods of achieving these objectives. The first basic method is to decimate the data which must be recorded or transmitted (where the term xe2x80x9cdecimate dataxe2x80x9d is used herein in the general sense of selectively eliminating part of a series of data sets) by selecting only vehicle position data which are estimated to have a sufficiently high degree of positioning accuracy. That is to say, information (referred to in the appended claims as xe2x80x9cindication informationxe2x80x9d) is derived in conjunction with each calculated vehicle position, which is used as a criterion for estimating the probable degree of accuracy of that position. Various forms of implementing such indication information can be envisaged, as described in the following.
The second basic method is to vary the precision to which a traversed route is expressed (i.e., the number of obtained positions per unit of distance) in accordance with distance from the current position or the terminating position of the route. This can be done by dividing a traversed route into a plurality of sections, utilizing the data of all of the vehicle positions constituting the most recent section of the route, then utilizing only data of periodically selected ones of the vehicle positions of the next section of the route, and so on. Thus, the precision to which the route is represented by the vehicle position data is maximized for the part of the route which is closest to the current position of the vehicle, while the total amount of data used to represent the traversed route is reduced.
More specifically, to achieve the first objective, according to a first aspect, the present invention provides a position data decimation apparatus and method for use in a vehicle navigation apparatus having position calculation means for deriving position data expressing successive positions of a vehicle in which the navigation apparatus is mounted, position recording means for recording the position data, whereby the position data decimation apparatus comprises position accuracy calculation means for estimating respective amounts of positioning error in the calculated vehicle positions, and position recording judgement means for determining whether or not each derived vehicle position is to be recorded by the position recording means, with that determination being based on the results obtained by the position accuracy calculation means.
In that way, by ensuring that only position data having a sufficiently high level of accuracy is stored in memory, for use in subsequently generating a display of the vehicle""s traversed route, the amount of data which must be stored in order to record a traversed route portion can be substantially reduced.
To achieve the first objective, according to a second aspect of the invention, rather than using position accuracy calculation means for estimating the accuracy of obtained positions, heading direction accuracy calculation means is used to estimate the accuracy of heading directions corresponding to respective derived positions of the vehicle. If the heading direction that is associated with an obtained vehicle position is judged to be of insufficient accuracy, then the position recording judgement means prevents the corresponding position data from being recorded.
To achieve the first objective, according to a third aspect of the invention, which is applicable to a vehicle navigation apparatus that obtains absolute vehicle position data based upon information supplied from navigation satellites such as GPS satellites, satellite number calculation means is used to estimate (for each obtained vehicle position) the number of satellites from which information is obtained. If the total number of satellites is judged to be insufficient, then the position recording judgement means prevents the corresponding position data from being recorded.
To achieve the first objective, according to a fourth aspect of the invention, which is applicable to a vehicle navigation apparatus that obtains absolute vehicle positions data based upon information supplied from navigation satellites such as GPS satellites and relative positions based on information from relative position sensors, only the absolute positions are recorded.
To achieve the second objective, according to a fifth aspect, the invention provides a position data decimation method and apparatus for use in a vehicle navigation apparatus having position calculation means for deriving position data expressing successive positions of a vehicle in which the navigation apparatus is mounted, position recording means for recording the position data, and position transmission means for transmitting the recorded position data by radio, whereby the position data decimation apparatus comprises position accuracy calculation means for estimating respective amounts of positioning error of the vehicle positions, and positioning error receiving means for recording the positioning errors.
If it is subsequently required to transmit data expressing a part of the traversed route, then the requisite recorded position data and accompanying positioning error values are read out and supplied to position transmission judgement means for determining whether or not each derived vehicle position is to be transmitted by the position transmission means, with that determination being based on the positioning error values.
In that way, by ensuring that only position data having a sufficiently high level of accuracy are transmitted by radio, expressing a portion of the vehicle""s traversed route, the amount of data which must be transmitted can be substantially reduced, and hence the route data can be transmitted within a shorter period of time.
To achieve the second objective, according to a sixth aspect of the invention, rather than using position accuracy calculation means for estimating the accuracy of obtained vehicle positions, heading direction accuracy calculation means is used to estimate respective amounts of direction error in vehicle heading directions that are obtained together with the vehicle positions and are recorded. If an emergency occurs, the recorded vehicle positions and corresponding heading direction error amounts are read out. If the heading direction that is associated with an obtained vehicle position is judged to be of insufficient accuracy, then the position transmission judgement means prevents the corresponding position data from being transmitted.
To achieve the second objective, according to a seventh aspect of the invention, which is applicable to a vehicle navigation apparatus that obtains vehicle position data based upon information supplied from navigation satellites such as GPS satellites, satellite number calculation means is used to estimate (for each obtained vehicle position) the number of satellites from which information is obtained, with that number being recorded, and subsequently read out together with the corresponding vehicle position in the event of an emergency. For each of these vehicle positions, if the total number of satellites is judged to be insufficient, then this considered to indicate that the accuracy of the obtained position information is insufficient, and the position transmission judgement means prevents the corresponding position data from being transmitted.
To achieve the second objective, according to an eighth aspect of the invention, which is applicable to a vehicle navigation apparatus that obtains vehicle position data based upon information supplied from navigation satellites such as GPS satellites, the determination as to whether or not an obtained vehicle position is to be transmitted in the event of an emergency is made on the basis of whether or not the vehicle position is an absolute position, i.e. has been obtained directly based on a source of absolute position information such as GPS information.
To achieve the second objective, according to a ninth aspect of the invention, the degree of precision of the route data is increased for a portion of the traversed route which is close to a desired final position, by comparison with portions of the traversed route which are more distant from the desired final position. Specifically, each of the derived vehicle positions is recorded in conjunction with the distance which the vehicle has moved since the preceding position was obtained. If it is subsequently required to transmit a portion of the traversed route, a set of successively obtained positions which extend to a specific cumulative distance from the current position, are each transmitted. A second set of successively obtained positions (extending from the farthest position of the first set), are then selected, and only a proportion of these are transmitted, for example every second position. This principal can be extended to one or more other sets of successively obtained positions, with the proportion of the contents of each set that are actually transmitted being reduced in accordance with distance from the final position of the vehicle. In that way, the final portion of the traversed route can be expressed with a high degree of precision by the transmitted data, while ensuring that the total amount of transmitted data can be reduced.